A navigation system may be used to navigate various things, such as navigating the direction of a drill bit to create a borehole or navigating the direction of a rover.
For several reasons, it is essential to accurately monitor and navigate the direction of the drill bit to create a borehole where desired. One reason is that it is expensive to drill a borehole at a cost of about $500,000 per day. Another reason is that it may be necessary by law for an oil rig to log the location of its boreholes at regular intervals such that the oil rig can be properly monitored.
Many prior systems have attempted to accurately and efficiently monitor the location of the drill bit to determine its location, but each system has had limitations. For example, the internal diameter of a drill pipe may not be large enough to fit the optimal number of typical navigation sensors. To overcome this obstacle, one prior system removes the drill bit from the borehole and lowers a monitoring tool down the borehole to determine its current location. A disadvantage of this system is that it is costly to stop drilling and spend time removing the drill bit to take measurements with the monitoring tool.
In another prior system, single-axis accelerometers are used to determine the vertical orientation of the drill bit. A system such as this, however, does not provide the drill bit's orientation relative to north, which is necessary to determine the full location of a borehole: a system that uses accelerometers is typically only adequate if the oil rig is going to drill a vertical borehole, since an accelerometer system cannot determine north.
In other prior systems, a magnetometer is used to determine the magnetic field direction from which the direction of north is approximated. However, systems such as these must make corrections for magnetic interference and use of magnetic materials for the drill pipe. Additionally, systems that rely only on magnetometers to determine north can suffer accuracy degradation due to the Earth's changing magnetic field.
One prior system uses only a single gimbal for all sensors. However, this system does not allow simultaneous estimation of all sensor biases nor the estimation of the north and the vertical for all borehole orientations. Other systems have used gimbals within a gyro sensor, but this does not provide all axes of observability.
Aside from using navigation systems for drilling boreholes, a navigation system may also be used to navigate the direction of a rover, which may include a land or subterranean rover or a roving person. A preferred navigation system for a rover is small in size and weight because of the limited carrying capacity of the rover. Typical navigation systems, such as the global positioning system (GPS) or other radio navigation aids are not always suitable for many rover applications such as when a land rover or roving person traverses a tunnel, a building, under foliage, in the urban canyon, operates in the presence of jamming or interference, or on the moon, Mars, or another celestial body.
Prior navigation systems have included the use of a small “strapdown” inertial navigation system that does not use gimbals to navigate. If the rover could constantly receive a radio navigation signal, it would be preferable to use a navigation system that includes a strapdown inertial navigation system since it provides a small system that only has to navigate between frequent radio navigation updates. However, inertial navigation strapdown systems typically suffer from long term error growth and thus cannot be used to navigate for long periods of time without updates.
The use of gimbals in an inertial navigation system is desirable to calibrate the sensors and to compensate for the sensor biases such that the system can accurately determine attitude and position. An inertial navigation system using gimbals may be more accurate by a factor of 100 compared to a non-gimbaled strapdown inertial navigation system. Moreover, an inertial navigation system that uses two or more gimbals only requires the sensors to be stable for a few minutes, rather than for days, in comparison to an inertial navigation system that doesn't use gimbals.